This invention relates to constant current sources and more particularly to a constant current source circuit having a substantially zero temperature coefficient in the temperature range from -50.degree. C. to +125.degree. C.
In many electrical circuits and systems there is a requirement for a current source which supplies a constant current that is relatively independent of the voltage applied thereto and is also relatively independent of the temperature of the operating environment. For instance, in a monolithic integrated operational amplifier circuit, a constant current source may be used for biasing the amplifier. As the bandwidth of the operational amplifier is a function of the bias current, it is very important that the biasing current be substantially constant over temperature variations. If the biasing current should vary with temperature, so will the bandwidth of the amplifier, which is undesirous.
One such prior art constant current source having temperature compensation is employed in the MC3503, "Quad Low Power Operational Amplifier", manufactured by Motorola, Inc., and illustrated in volume 6 of the "Semiconductor Data Library", pages 8-546 to 8-551. The constant current source comprises: a field effect transistor; a diode; three transistors; and a resistor. Although the temperature coefficient is very good, the number of components not only increase cost of the amplifier to the consumer, but also require a good portion of the chip die area to be fabricated therein. Thus, a need exist for a rather simple and less expensive device for providing a constant current source having a substantially zero temperature coefficient.
Furthermore, there is a prior art current regulator which comprises a junction field effect transistor having its source connected to its gate and which is connected between the base electrode and the emitter electrode of a bipolar transistor. The collector of the bipolar transistor serves as one of the electrodes of the current regulator. A resistor is connected between the emitter of the transistor and a second terminal of the current regulator. A zener diode is connected between the base of the transistor and the second terminal of the current regulator. The current regulator of the prior art provides a current which is relatively independent of the voltage applied across its terminals, but which vary substantially with the ambient temperature. In particular, the current therein depends heavily on the temperature variations of the emitter resistor, the base to emitter forward bias voltage of the bipolar transistor, and the voltage across the zener diode.
The present invention solves the shortcomings of the above prior art devices by providing a temperature compensated constant current source of simple construction and which has a temperature coefficient which is substantially zero.